This invention relates to a control system for a position type control. The control system controls the position type control pneumatically.
In such systems, the positioner is the main controlling device. In the past, these systems have had a bellows unit which employs the principle of balanced operation by spring loading the bellows unit to the same force as that exerted by the control instrument air pressure. The bellows unit is connected to a pilot valve. When the control instrument air pressure is increased, the bellows move against the balance spring. This opens the pilot valve in the direction to allow the air supply to flow into the back of the cylinder, pushing the piston forward, which thereby increases the output speed of the motor drive. As the piston moves forward, a cam which is mounted on the piston rod moves the positioning lever outwardly. This compresses the balance spring through the slider assembly and the bellows lever assembly. The piston moves forward until the balance spring is compressed to give sufficient spring load to balance the increased control instrument air pressure. When this balance is obtained, the bellows will move back to the neutral position and close the pilot valve. This stops the air supply flow to the back of the cylinder and the piston ceases to move.
When the control instrument air pressure is decreased, this operation is reversed. The control instrument air pressure is lower than the balance spring pressure. This allows the balance spring to move the bellows in the opposite direction, which opens the pilot valve to allow air to escape from the back of the cylinder. The back air pressure in the cylinder and the balance spring force will cause the piston to move backward, which will decrease the output speed of the motor drive. As the cam moves with the piston rod, it allows the positioning lever to move in, decompressing the balance spring. The piston will move until the balance spring pressure balances the control instrument air pressure. The bellows will assume the neutral position closing the pilot valve and stopping the piston.
The above-described known control system is a good control, but is not as positive in slowing the drive as is desired. Furthermore, since it depends upon the spring force to move the positioner assembly back and to thereby decrease the speed of the motor drive, it is necessary to change the spring when a greater force is desired or necessary.